Cathode for electrolytic erosion of metal



March 1, 1966 R. D. HALVERSTADT ETAL 3,238,114

CATHODE FOR ELECTROLYTIC EROSION OF METAL Filed June 6, 1960 5 Sheets-Sheet 1 POWER SUPPLY UNIT g ua

March I, 1966 R. D. HALVERSTADT ETAL CATHODE FOR ELECTROLYTIC EROSION 0F METAL Filed June 6, 1960 5 Sheets-Sheet 2 Jig 5&6

N V EN TORS March 1, 1966 R. D. HALVERSTADT ETAL GATHODE FOR ELECTROLYTIC EROSION OF METAL Filed June 6. 1960 5 Sheets-Sheet 4 (#425 J J a, mad

March 1, 1966 R. D. HALVERSTADT ETAL 3,233,114

CATHODE FOR ELECTROLYTIC EROSION OF METAL 5 Sheets-Sheet 5 Filed June 6, 1960 ooh United States Patent 3,238,114 CATHODE FOR ELECTROLYTIC EROSION OF METAL Robert Dale. Halverstadt, Marblehead, Mass., Donald Archer Stewart, Springdale, Ohio, and Charles Joseph Hunt, Taylor Mill, Ky., assignors to General Electric Company, a corporation of New York Filed June 6, 1960, Ser. No. 34,334 6 Claims. (Cl. 204-224) This invention relates to an improvement in the art of removing metal from a workpiece by electrolytic erosion. The common or shop term for two forms of this art is electrolytic grinding and electrolytic drilling, regardless of the amount of actual mechanical grinding or drilling which takes place.

Until comparatively recent years, all metals, including commercially available alloys, could be formed and finished by conventional cutting tools and grinders. However, the development of high-performance power producing apparatus such as gas turbines brought about the invention and production of a class of metals which are commonly called super-alloys. The efliciency of gas turbines generally increases very substantially with the increase of the temperature of the gases. produced by the combustion of the fuel. Hence, the development of this type of turbine required materials suitable for structural parts which would maintain adequate rigidity in the presence of heavy deforming stresses at temperatures such as 1500 F. and above. Further, for these turbines to have any substantial life, it was and is necessary that the critical structural parts be very hard and resistant to erosion and corrosion by the high velocity streams of hot gases. The so-called super-alloys, as materials, were the answer to the exacting requirements of the gas turbine engine. The solution of the problem of suitable materials, however, posed the new problem of their fabrication. The cathode of this invention was particularly developed to alleviate this problem.

The super-alloys are very resistant to all common metal shaping operations. On a practical production basis, complex articles made from these alloys cannot be machined by edge-cutting tools or by grinding. In the typical gas turbine, there is need for super-alloy parts of intricate and thin cross sections which must be held to close dimensional tolerances, such as one ten thousandth of an inch (.0001). The fabrication of parts of thin cross section from available super-alloy stock often requires removal of far more material than remains as the finished part.

At present, the super-alloy parts are fabricated from the super-alloy stock by conventional grinding with a vitrified aluminum oxide grinding wheel. In many cases, it is not feasible to remove more than one thousandth of an inch (.001) from the surface of the workpiece in a single pass. Many fabricating operations require the removal of metal from the workpiece to a depth of onehalf inch or more. Hence, this fabrication with the conventional grinding wheel is highly time-consuming and tedious, to say the least. Further, this grinding tends to produce stresses and actual flaws at the surface of the workpiece which may become the focus of failure of the part. If, on inspection, a flaw appears in a part it must be rejected. This necessary precaution results in the production of a substantial quantity of very expensive scrap. Further, such conventional grinding often introduces a residual stress which is not visually apparent but which causes premature failure during operation.

To overcome the slow rate of grinding with vitrified aluminum oxide wheels, the use of diamond grinding wheels has been tried. For simple, uniform cutting, diamond wheels may be useful. However, diamond 3,238,114 Patented Mar. 1, 1966 wheels have never proved feasible for pure abrasive grinding of anything but sintered carbide, ceramic and the like. They load too easily with softer materials. tion, the cost of diamond wheels is many times greater than such wheels as vitrified aluminum oxide.

However, even if diamond wheels were feasible for pure abrasive grinding, they would have to be reconditioned after very limited usage because the hardness and toughness of the super-alloy is such that even a diamond grinding wheel has limited useful life. Reconditioning a metal bonded diamond wheel is virtually impossible where intricate shapes are required. Therefore, intricate electrolytic grinding using such wheels is virtually impossible.

One object of the present invention is to improve the art of shaping and finishing metal workpieces falling within the super-alloy category and particularly to provide a special tool for this purpose which combines the electrical advantages of a metal electrode with the mechanical advantages of a dielectric material. Although the invention and its advantages are described and explained primarily in relation to the problems which were created by the introduction of the super-alloys, the cathodes of this invention may also be used for shaping or dressing metal bonded carbide cutting tools, metals too hard for conventional machining and grinding, and softer or ordinary metals, if expedient.

Another object is to provide an improved cathode for rapid electrolytic and mechanical erosion of metal, which cathode combines low cost with good dressability, particularly in complicated intricate shapes.

Other features and objectives of the invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a diagrammatic view of grinding apparatus utilizing the invention;

FIG. 2 is, a fragmentary view of an element shaped by grinding in accordance with the invention;

FIG. 3 is a perspective view of an element shaped by grinding in accordance with the invention;

FIG. 4 is a cross-sectional view of a grinding wheel and electrolyte supply utilizing the invention;

FIGS. 5 and 6 are diagrammatic cross-sectional views of grinding wheels and workpieces illustrating the ope-ration of the invention;

FIG. 7 is a cross-sectional view of an alternative type of grinding wheel;

FIG. 8 is an end elevational view of the grinding wheel of FIG. 7;

FIG. 9 is a diagrammatic view partly in section of apparatus for impregnating a grinding wheel; and

FIG. 10 is a diagrammatic view partly in section of alternate apparatus for impregnating a grinding wheel.

In one form, this invention provides a cathode composed of material which is a non-conductor, or an inadequate conductor of electricity, but which is permeated by pores collectively constituting a continuous system. These pores are impregnated with a material which is or can be made to be a good conductor of electricity. This impregnation may be complete, with the pores fully filled, or partial, with the walls of the pores merely coated enough to provide a continuous electrical path through the material. In either case, the resulting cathode is characterized by mechanical properties which can only be found in dielectric materials and electrical conductivity akin to that of metals.

In electrolytic grinding and electrolytic drilling, a direct current is passed through an electrolyte disposed as a film between the workpiece and a complementary electrode corresponds functionally with the cutting or abrad- In addiing tool customarily used for removing metal from a workpiece. This electrode may or may not provide a mechanical removal of metal coincidentally with the electrolytic erosion, depending upon the composition of the electrode and the manner in which it is applied to the workpiece. Regardless of whether or not abrasive metal removal takes place, the rotating electrode scrubs the surface being cut and removes any passivating film left as a product of electrolysis.

Direct current, which is passed through this film of electrolyte from the anode workpiece to the cathode tool, tends to remove metal from the anode and plate it on the cathode. The chemistry of the operation is no different from that of an ordinary electroplating process except that the electrolyte is constituted to precipitate the metal ion removed from the workpiece so that it will not be plated or deposited on the cathode. The principle of the process is sometimes referred to in the art as de-plating.

The cathode of this invention permits the use of high current densities in the film of electrolyte between the cathode and the workpiece and also permits supplementation of electrolytic erosion by the mechanical action of the cathode on the workpiece as a cutting or grinding medium as well as an improved scrubber. By appropriate selection of the material of the body of the porous cathode, the mechanical action may be varied from a light rubbing or polishing to substantial grinding or cutting. In any case, the cathode of this invention combines any desired physical properties of a nonmetal with the conductivity of a metal.

The electrolyte used in conjunction with the electrode of this invention may be applied by spout to the interface between the cathode and workpiece, but it is considered preferable to apply the electrolyte through the porous system of the cathode to assure an adequate supply of electrolyte in the zone of erosion. This feed of the electrolyte through the cathode may be accelerated by feeding the electrolyte to the cathode under pressure or by centrifugal force if a rotating wheel-cathode is used. If the flow of electrolyt through the cathode is inadequate, the supply of electrolyte may be supplemented by a spout discharge.

The preferred cathode of this invention is a vitrified aluminum oxide grit grinding wheel which may be of any standard configuration and which is at least partially impregnated with an electrically conductive material or electrically conductive ink, of the type now employed in the fabrication of printed circuits. A preferred ink is one which suspends finely divided silver oxide in a suitable vehicle. After the wheel is impregnated, the silver oxide is reduced to metallic silver by heat treating at a temperature at which silver oxide reduces. Electrically conductive refers to the condition of the material or ink when the cathode is ready for use. The coating of ink on the walls of the pores of the wheels provides a relatively low-resistance electric connection between the hub of the wheel and the circular margin of the wheel. The electrolyte may be delivered to the -point of use through the wheels continuous system of .pores.

Further, a vitrified aluminum oxide grinding wheel is itself an excellent grinding tool in abrasive capacity and wear resistance. While reference has been made to the preferred vitrified aluminum oxide grinding wheel, vitreous bonded carbide wheels such as of silicon carbide impregnated with electrically conductive material will perform, in many situations, equally as well as the aluminum oxide wheel.

Vitrified aluminum oxide grinding wheels are in common use and relatively inexpensive. The shops which perform grinding operations are accustomed to their use and are familiar with the techniques of dressing them. The partial impregnation with the conductive ink does not detract from the mechanical properties of these wheels but imparts to them an electrical conductivity which converts an excellent grinding wheel into an excellent cathode for electrolytic erosion. The combination of good abrasive properties and good electrical properties permits this bi-functional tool to be used to produce variable ratios of electrolytic action to grinding action, which extend over a substantial range.

The present invention and its advantages are explained herein primarily with a reference to the partially impregnated vitrified aluminum oxide grinding wheel which is the preferred embodiment of the invention. While this wheel, and others embodying its principle, may be used on any metal desired, the most important utility of the wheel is best explained by reference to its most exacting condition of use.

In general, the cathode of this invention makes available a production tool capable of shaping metal to the most complicated configurations at a production cost many times less than the cost of any other method and with any other cathode. It makes possible close tolerance from grinding heretofore unobtainable because of the diificulty, if not impossibility, of dressing metal bonded diamond wheels to complicated shapes.

In the form of a grinding wheel, this invention is very effective for removing metal from super-alloy workpieces. Cuts of a quarter of an inch or deeper may be taken on the preliminary or roughing passes and the tolerance held to one ten thousandth of an inch (.0001) on the finish pass. In other words, the removal of metal on the roughing pass can be many times greater than the removal of metal on a single pass by a conventional grinding wheel. Thus a sizable saving in time and grinding costs is achieved through the present invention.

In using the impregnated vitrified aluminum oxide grinding wheels of this invention, it has been found that the maximum cuts do not always maintain the allowable dimensional tolerances. The maximum cut is obtained by the use of relatively high-current density in the zone of the electrolytic erosion. At these current densities, the removal of metal is effected almost entirely by electrolytic erosion and the wear on the wheel is almost nil.

However, the degree of electrolytic erosion is not always reliably precise to the required degree. It is hypothesized that the cations provided by the electrolyte combine with the metal or attack the metal workpiece and that the grit merely sweeps away the reaction products and presents fresh electrolyte and a workpiece surface uninhibited against further attack.

The difficulty of holding dimensions within the desired tolerance on a heavy cut is explained by the assumption that the electrolytic action proceeds in a zone very slightly in advance of the zone of mechanical grinding by the grit. Thus, the resulting surface is established by the electrochemical action of the electrolyte, which may be variable instead of by the mechanical action of the grit, which is relatively precise.

The cathode of this invention, may be used to make a finish pass which holds dimensions within the close tolerances required. This is done by increasing the speed or by decreasing the voltage on the finish pass. Such action decreases the current density and the resulting erosion to such a degree that the grit mechanically grinds the workpiece and establishes its final or finished surface.

However, this mechanical grinding is not unaided by electrolytic erosion and, in fact, the aluminum oxide grinding wheel could not perform a comparable grinding operation on the normally refractory surface of a superalloy in the absence of the electrolytic erosion. This unexpected grinding action is explained by the hypothesis that the cations of the electrolyte attack and weaken or roughen or deteriorate the surface of the workpiece so that the grit of the Wheel is not working on the intact, smooth surface of the super-alloy. Such eroding-grinding produces less stress in the workpiece than grinding with other types of wheels and fewer flaws result.

Therefore, the method of this invention comprises a finish pass wherein a metal surface modified by electrolytic attack is ground mechanically. It also comprises the two-step process of making at least a roughing pass, and then a finish pass, the resulting surface being established by electrolytic erosion on the roughing pass and by mechanical grinding plus electrolytic erosion on the finish pass.

A wheel formed in accordance with the invention is employed in the grinding of a workpiece by apparatus diagrammatically illustrated in FIG. 1. There a wheel 20 is mounted on any suitable operating mechanism 21. The mechanism includesmeans for reciprocating the wheel '20 longitudinally and transversely with respect-to a workpiece 22. The mechanism also includes means for rotating the wheel 20 at rates of speed which may be varied as required by the conditions of the particular grinding operation being performed.

The workpiece 22 ismounted on a conductive fixture 23 which is connected by a conductor 24 as an anode to the positive side of a direct current power supply 25.

A'con-ductor 26 connects the negative side of thepower supply to the wheel 20 so that the wheel 20 will become a cathode in the electrolytic circuit.

A reservoir 27 of electrolyte is connectedby a tube 28to a cavity in the central portion of the wheel 20. A valve 29 is provided in tube 28 to regulate the flow of electrolyte to the wheel 20.

A suitable electrolyte catch basin (not shown) is associated with the fixture 23. A return tube 30 is provided -for'conveying electrolyte through a filter 31 and pump 32 back to reservoir 27.

As will be discussed in detail below, an objective of the invention resides in the providing for the variation in the ratio of electrolyte grinding to mechanical grinding. To

this end we provide means for varying the voltage im- "pressed on the electrodes (workpiece 22 and wheel 20),

to 6000 surface feet per minute during the grinding operation. In the case of electrolyte introduced at .the wheel'hub, the rotation of the wheel imparts a centrifugal force to the'electrolyte which drives the electrolyte through the. porous wheel. In general, the electrolyte should be supplied at a rate substantially equal to the rate at which it is removed from the him or center of the wheel by the centrifugal action ofthe rotating wheel. However,

the electrolyte can also be supplied by an appropriately located nozzle directed at the wheel surface.

'The electrolyte may be any suitable solution which will permit'the efiicient electrolytic erosion of .a workpiece. As an example of a satisfactory electrolyte, we have used an aqueous solution of sodium chloride and boric acid.

" Preferably, the electrolyte should contain a reagent which will combine with the metal ions deplated from the workpiece to form a precipitate which will prevent the metal from being plated onto the wheel. The particular reagent or reagents employed will depend in part upon the constituents metals of the alloy being processed. If the alloy is predominantly iron, sodium formate can be used satisfactorily as a reagent. Alternatively, the deplated metal ions can be separated out of the solution by passing the electrolyte through a precipitation or sludging tank or through an ion separator.

The electrolyte can be recycled through the filter 31 by the pump 32 in order to assure that the electrolytic solution is free from solids which would tend to clog the interstices of the grinding wheel.

It should be understood that the apparatus illustrated in FIG. 1 is merely by way of example. Any suitable structure for mounting the grinding wheel and workpiece and for supplying electrolyte to the wheel and current between the wheel and workpiece can be employed without departing from the spirit and scope of the present invention.

In operation, a voltage of 16 volts, for example, is applied between the wheel and workpiece, which will result in a current through the electrolyteof 30-140 amps, depending upon the size of the wheel and the depth of cut. However, as much as 600 amps have been used successfully. The wheel is driven at thedesired speed and the electrolyte is supplied to the wheel. Asthewheel .passes along the workpiece, the electrolytic action will cause the removal of metal from the workpiece following the contour of the wheel. During-theinitial' grinding, the material is removed fromthe workpiece almost entirely electrolytically.

Even though there may be no metal removal by abrasive or mechanical tgrinding therotating electrode wheel scrubs the-surface being cutand removes any;passivating film formed. This scrubbing leaves a surface which is clean andideal to promote electrolyticaction.

In the process of forminga shaped-part from a superalloy metal blank, as many roughing electrolytic grinding passes are made as areneoessary for the rough finishing. of the part. Thereafter, either the voltage applied vto the electrolytic circuit is decreased or alternatively, the relative speed between. the travel of the .wheel and the anodic workpiece is increased. The effect of such variation of either of these parameters is such as to increase the relative-mechanical-grinding actionas compared to the electrolytic metal removal. For simplicity. and economy, the voltage variation method. of changing. the elec- -trolytic grinding to mechanical grindingvratio is referred.

This can be accurately monitored by measuring power increase to turn the-wheel spindle. Any power increase denotes an equivalent mechanical removal increase.

FIGS. 2 and 3 are illustrative of the type of cross sections which can be formed by the process of the present invention. A shaped member 35 in'FIGa 2 wasformed as shown by. the electrolytic grinding process of themesent invention from a solid bar of approximately 1% inch stock. The metal was a super-alloy designatedas M-25 2,

which is constituted, in percent by weight, as follows:

C .1 to .2.

Cr 18.t0 20.

Co 9 to 11.

Mo 6 to 11.

Ti 2.25. .to 2.75.

Al .25 to 1.25.

-Ni Balance.

Similarly, a circular member 36 was formed from 1 inch stock. In both members 35 and-36 it is important to note the thin fin configurations can be formed Within as close tolerances as 0.0001 inch through the process, of the present invention because little or no mechanical force is exerted.

FIG. 4 illustrates one form of a wheel constructed in accordance with the present invention. A wheel 40,

which can be a standard vitrified aluminum oxide grinding wheel impregnated with a conductive substance in the manner described below, has a central opening 41. A metallic adapter 42 has an annular flange 43 adapted to bear against one side 44 ofthe wheel. .A hollow stub 45, which is an integral part of the adapter 42, passes through the opening 41 .of the grinding wheel and is threaded as at 46 to receive a nut.47 by which the wheel 40 is clamped to the adapter. The adapter has a hollow stub 48 which receives'a shaft 49. The shaft 49 is connected to a motor for rotating the wheel at the proper speed for grinding. The adapter 42 is fixed to the shaft 49 by means of a set screw 50. The electrolytic voltage 7 may be applied to the wheel through a brush 51 which is maintained in engagement with the surface of stub 48.

The adapter Stub 45 has a plurality of circum-ferentially spaced orifices 52 through which electrolyte passes from the adapter stub to the wheel 40. The wheel 40 may be provided with an annular recess 53 which is disposed adjacent the orifices 52 so as to form a pocket to receive a reservoir of electrolyte, the electrolyte flowing from the pocket 53 through the pores in the wheel 40 out to the periphery 54 of the wheel. The electrolyte is supplied to the adapter through a tube 55 which projects into the hollow stub 45 and terminates adjacent the orifices 52. The tube 55 is connected by a suitable hose 56 to a supply of electrolyte. A packing between the tube 55 and the stub 45 may be provided, however such packing is not necessary as long as the supply of electrolyte through a tube 55 does not exceed the ability of the grinding of the wheel 40 to remove the electrolyte discharged into the stub 45 by means of the centrifugal force which drives the electrolyte through the orifices 52 and throughout the wheel.

Because the pores in the wheel 40 will be located at random throughout the wheel, the electrolyte will tend to disperse uniformly throughout the cross section of the wheel. Thus, there will be uniform distribution of electrolyte along the abrading surface 54 which is in contact with the metal.

FIGS. and 6 illustrate two possible configurations of the type of wheel illustrated in FIG. 4. In FIG. 5 the edge of the wheel is generally triangular in cross section as indicated at 57. The removal of metal will leave a similar concave groove 58 which is of the same triangular section.

In FIG. 6 the edge of the wheel is of concave semicircular section as indicated at 59. The workpiece subjected to the action of the wheel of FIG. 6 will be formed with a semicircular ridge 60. Although the grinding surface of the various forms of the wheels shown herein are illustrated as simple shapes, it is to be understood that such surfaces can be dressed to any shape, simple or complex, to result in correspondingly shaped ground surfaces on a workpiece.

FIGS. 7 and 8 illustrate a different form of wheel from that illustrated in FIGS. 4, 5 and 6. In FIG. 7 the abrasive portion of the wheel is a comparatively thin walled cylinder 61. By way of example, the cylinder 61 may have a wall thickness of approximately one inch for a wheel of fourteen inches outside diameter. An adapter 62 secures the cylinder to a rotatable shaft 63. The adapter comprises a plate 64 in engagement with one edge 65 of the cylinder and a plate 66 in engagement with the other edge 67 of the cylinder. A foraminous cylinder 68 is located between the plates 64 and 66, and is secured to plate 66 such as by welding. The plate 64 is threaded on the cylinder 68 as at 69.

The foraminous cylinder performs the dual function of strengthening the abrasive cylinder by forming a backing therefor and serves to distribute the electrolyte uniformly throughout the internal surface of the abrasive cylinder.

The plate 64 has an internally threaded central opening 70 to which a hollow stub 71 is threaded. The stub 71 is connected to a fitting 72 which is supplied with electrolyte from a tube 73. The hollow stub 71 is rotatably secured in the fitting 72 with a suitable packing which will permit the rotation of the stub 71 with respect to the fitting 72.

During the operative rotation of the wheel 61, electrolyte flows from tube 73 through the hollow stub 71 and into the cavity formed by the abrasive cylinder and the foraminous cylinder. Centrifugal force developed by the rotation of the assembly, forces the electrolyte through the pores of wheel 61 and substantially uniformly onto the outer surface of the wheel.

As indicated above, an important aspect of the present invention is the provision of means for changing a normally non-conductive wheel to a wheel of sufiicient conductance to form a cathode in an electrolytic circuit. The wheel may be of standard manufacture and of the type which is naturally porous. The vitrified aluminum oxide grit wheel is ideally suited for this purpose. In one form, the method of rendering such a wheel conductive comprises initially impregnating the Wheel with a material which is conductive or can be made conductive. Such material can be carried into the interstices of a cathode, such as a wheel, by a fluid such as a liquid or gas. The only limitation on the size of material used to impregnate the cathode is that it be small enough to penetrate the pores or openings in the cathode and not be filtered out. Thereafter the wheel is heated to drive off or decompose the fluid carrier, such as a solvent, and, when appropriate such as with the use of a thermally reducible oxide as silver, to make the material conductive. After such heating, the walls of the pores are uniformly coated with a conductive substance. In another form for use with electrolyte fed externally, the method of rendering the cathode conductive can merely consist of impregnating the cathode with a conductive material without regard to the creation of electrolyte-carrying pores.

Many suitable impregnating materials are available. Although they are sometimes referred to herein as electrically conductive materials or electrically conductive inks it is to be understood that such reference is intended to include in its meaning those materials which are electrically conductive as well as those materials which can be made electrically conductive, for example such as through heating to volatile or decompose a carrier or to reduce an oxide to its free metal.

One type is an ink made from silver pigment suspended in a suitable carrier. The preferred type is a silver pigment ink of the type sometimes employed in the manufacture of printed circuits because such inks impart'better conductivity to the cathode than a pure silver ink. This ink comprises a suspension of finely divided silver oxide in a resin carrier. The silver oxide of this ink will reduce to metallic silver at about 800 F. Trichlorethylene can be used as a solvent to thin the ink to approximately the consistency of water, a condition which has been found to be useful in the impregnation process.

More specifically, a preferred method of impregnating a wheel comprises the steps of first evacuating a chamber in which the wheel is located and thereafter introducing the conductive ink under pressure into the chamber so that the ink will flow freely and uniformly throughout the previously evacuated pores of the wheel. One form of apparatus for performing this method is illustrated in FIG. 9.

A cylinder 75 forms a chamber in which a grinding wheel is mounted on suitable supports 76 at the closed bottom wall 77 of the cylinder. A vacuum passageway 78 is formed in the wall of the cylinder and is connected by a fitting 79 and valve 80 to a line 81 which is in turn connected to a vacuum pump not shown. A liquid passageway 82 is also formed in the wall of the cylinder 75 and connected by a fitting 83 and valve 84 to a line 85. The line 85 is connected to a supply of conductive ink 86. A piston 87 is reciprocably mounted in the cylinder 75 and is actuated by a hydraulic ram 88.

The following is an example of a process for impregnating the wheels. In the apparatus shown, a vitrified aluminum oxide wheel of 2" x2"x1" made from particles of grit size or larger was placed in the chamber formed by the cylinder 75. The chamber was evacuated to approximately 28.5 inches of mercury vacuum to remove as much air from the pores of the wheel as possible. An ink including silver oxide suspended in a liquid carrier was thereafter admitted to fill the chamber. The hydraulic ram was then operated to drive the piston downwardly to apply a pressure of 3,000 psi. The thus impregnated wheel was removed from the apparatus and baked at a temperature, for example 800 F., to decompose and drive otI the liquid carrier and to cause the impregnation to become conductive by reduction of the silver oxide to metallic silver. The treated specimens were found to be completely conductive. The method described above will be particularly applicable to small diameter wheels.

The apparatus of FIG. is designed for the impregnation of larger diameter wheels. The apparatus comprises a plate 90 having an annular flange 91 projecting vertically to form a circular cavity 92. The bottom wall of the cavity is provided with point supports 93 to receive the wheel to be impregnated. The cavity 92 is enclosed by a plate 94 and sealed and an O ring 95, the plate 94 being clamped to the flange 91 by bolts 96.

A vacuum pump not shown is connected by a line 97 through a valve 98 and fitting 99 to the cavity 92, the fitting 99 being connected to a bore in the plate 94. A pressure gauge 100 is also connected to the cavity 92 through a line 101 and fitting 102 threaded in a bore 103 in the plate 94. The pressure gauge is adapted to indicate vacuum as well as pressure, for example, up to 3,000 p.s.i. A hydraulic press 104 adapted to deliver a pressure, for example, of 3,000 p.s.i., is connected through a line 105, valve 106 and fitting 107 to the cavity 92. The fitting 107 is connected to the cavity 102 through a bore 108 in the plate 94. A conductive ink reservoir 109 under atmospheric pressure is connected through a line 110, valve 111 and fitting 112 to the cavity 92. The fitting 112 is connected to the cavity 92 through the passageway 113 formed in the plate 90.

Again, by way of example, a grinding wheel indicated at 114 was rendered conductive by the following process. The grinding wheel 114 was positioned on the supports 93 in the cavity 92 and the plate 94 was applied by the bolts 96. The cavity was first evacuated to approximately 28.5 inches of mercury vacuum to remove air from the pores of the wheel. Thereafter valve 111 was operated to admit silver ink under atmospheric pressure to fill the cavity 92. The valve 111 was closed and valve 106 operated inconjunction with the hydraulic press 104 to place the cavity 92 under a pressure of 3,000 p.s.i. The applied pressure caused a thorough impregnation of the wheel with the silver ink. Thereafter, the wheel 114 was removed and baked at 800 F. to decompose and drive off the liquid carrier and to cause the impregnation to become conductive.

A particular advantage in using silver as an electrical carrier in the cathode of this invention is that any oxidat io'n of the silver during operation can be removed by simply heating the cathode to reduce the oxide to metallic silver.

Although the present invention has been described in connection with specific examples, it is not intended to be limited except as set forth in the claims which follow.

What is claimed is:

1. A grinding wheel for the electrolytic erosion of metal comprising: a body having an annular-shaped workengaging surface and a centrally located means adapted for rotation of said wheel on its axis, said body comprising abrasive grains bonded together by electrically non-conductive dielectric material, said abrasive grains and dielectric material being permeated by pores which collectively constitute a continuous system; a finely divided electrically conductive silver material disposed in the pores and forming a continuous electrically conductive path between a source of electric power and the workengaging portion of the wheel.

2. Means for the electrolytic erosion of metal comprising: a source of electric power, a cathode, means for supporting a workpiece adjacent said cathode and means for introducing an electrolyte into the area between said cathode and workpiece, said cathode comprising: dielec 10 tric material bonded together to form pores which collectively constitute a continuous system and an electrically conductive silver material coating the walls of the pores and forming a continuous electrically conductive path between the source of electric power and the work-engaging portion of the cathode.

3. An improved cathode for the electrolytic erosion of metal comprising: a dielectric, vitrified bonded, aluminum oxide grinding wheel permeated by pores which collectively constitute a continuous system adapted to transmit electrolyte to the surface of the wheel; and electrically conductive silver coating the walls of the pores forming a continuous electrically conductive path between a source of electric power and the work-engaging portion of the wheel, whereby the grinding wheel is rendered electrically conductive.

4. An improved grinding wheel for the electrolytic erosion of metal comprising: particles of dielectric material bonded together to define pores which collectively constitute a continuous system adapted to transmit electrolyte to the surface of the wheel; and sufficient electrically conductive silver material coating the walls of the pores to define a continuous electrically conductive path between a source of electric power and the work engaging portion of the wheel, the conductive material being insuflicient to close the pores of the wheel.

5. A cathode for the electrolytic erosion of metal, the cathode comprising: a hollow cylinder having a side wall made of particles of a dielectric material bonded together to define pores which collectively constitute a continuous system; and electrically conductive silver material disposed in the pores defining a continuous electrically conductive path between a source of electric power and the work-engaging portion of the cathode; the hollow portion of the cylinder providing a reservoir for electrolyte.

6. A cylindrical grinding cathode for the electrolytic erosion of metal, the cathode comprising: a hollow cylin-- der having an outer side wall made of particles of a dielectric material bonded together to define pores which collectively constitute a continuous system; a driving, spindle connected with the cathode at the inner side wall of the hollow cylinder; and electrically conductive silver material disposed in the pores to define a continuous electrically conductive path between a source of electric power and the work-engaging portion of the wheel; the hollow portion of the cylinder providing a reservoir for electrolyte to be forced through the pores.

References Cited by the Examiner UNITED STATES PATENTS 2,474,502 6/ 1949 Suchy 20420- 2,773,844 12/1956 Carlson et al. 117160 X 2,798,846 7/ 1957 Comstock 204-143 2,813,808 11/1957 Hoyer 117227' 2,840,960 7/ 1958 Booth. 2,848,410 8/ 1958 Knuth-Winterfeldt 204-224 2,885,310 5/1959 Olsen l17227 2,905,605 9/ 1959 Keeleric et a1 204143 2,918,392 12/1959 Beller 117160 X 2,947,616 8/1960 Coes et al. 51-296,

FOREIGN PATENTS 1,219,463 12/1959 France.

748,485 5/ 1956 Great Britain.

765,101 1/1957 Great Britain.

342,113 12/ 1959 Switzerland.

WINSTON A. DOUGLAS, Primary Examiner.

JOHN H. MACK, Examiner.

D. R. JORDAN, Assistant Examiner. 

1. A GRINDING WHEEL FOR THE ELECTROLYTIC EROSION OF METAL COMPRISING: A BODY HAVING AN ANNULAR-SHAPED WORKENGAGING SURFACE AND A CENTRALLY LOCATED MEANS ADAPTED FOR ROTATION OF SAID WHEEL ON ITS AXIS, SAID BODY COMPRISING ABRASIVE GRAINS BONDED TOGETHER BY ELECTRICALLY NON-CONDUCTIVE DIELECTRIC MATERIAL, SAID ABRASIVE GRAINS AND DIELECTRIC MATERIAL BEING PERMEATED BY PORES WHICH COLLECTELY CONSTITUTE A CONTINUOUS SYSTEMF A FINELY DIVIDED ELECTRICALLY CONDUCTIVE SILVER MATERIAL DISPOSED IN THE PORES AND FORMING A CONTINUOUS ELECTRICALLY CONDUCTIVE PATH BETWEEN A SOURCE OF ELECTRIC POWER AND THE WORKENGAGING PORTION OF THE WHEEL. 